The present invention relates to a method for the powder metallurgical production of structural parts of great strength and hardness from silicon-manganese or silicon manganese-carbon alloyed steels.
Since the strength of unalloyed sintered iron is relatively low (even with the greatest density, only about 300 MPa are realized), higher strength requirements can be met only with alloying type measures.
The elements Cu, Ni, Mo, P and C are primarily used as alloying elements. A whole number of alloying elements used in melting metallurgy can be used only conditionally in powder metallurgy. These are alloying elements such as Cr, Mn, Si and Ti, which have high affinity for oxygen, and of which Si and Mn are of particular interest because of their favorable price and their long-term assured availability.
Si is known as a solid solution hardener having a great strengthening effect. In powder metallurgically produced Fe-based alloys, Si is also able to produce considerable increases in strength. See, Hoffman, G. Thummler, F., Zapf, G., 37 Sintering, Homogenization and Properties of Alpha-Phase Iron-Aluminum and Iron-Silicon Alloys", Powder Metallurgy, 3rd Europ. Powd. Met. Symp. 1971, Conf. Supplement, Pt. 1, pages 335-361. However, there are two drawbacks. Firstly, due to its high oxygen affinity, Si tends to form oxides in industrial sintering atmospheres. This could be countered by using an Fe-Si master alloy as the alloy carrier. See German Published Patent Application No. 1,928,930, entitled "Method for Producing Sintered Materials on an Iron Base". However, sintered steels produced according to this method are unsuitable for structural parts since intensive shrinkage occurs during the sintering process which has a negative effect on the bodies' retention of dimensions. Experiments have been made to compensate for the shrinkage by alloying in further elements such as Cu and Al. However, this is only partially successful and is additionally connected with loss of strength characteristics. See, Hoffmann, G., Thummler, F., Zapf, G. "Sintering, Homogenization and Properties of Alpha-Phase Iron-Aluminum and Iron-Silicon Alloys", ibid.
Mn has also found acceptance as an alloying element in powder metallurgy. Since it likewise has a great affinity for oxygen, special protective measures are required here as well. It is known, for example, to introduce Mn by way of carbidic master alloys. See German Pat. No. 2,456,781, entitled "Method for Producing Homogeneous Manganese Alloyed Sintered Steels." These master alloys are stable up to the range of the sintering temperature so that the alloying elements are protected against oxidation. However, since Mn itself is not a strong carbide former, these master alloys must inevitably contain carbide forming elements, such as, for example, Cr, Mo or V. These elements are very expensive which considerably increases the costs for the alloys. Moreover, the great hardness of the carbides results in increased tool wear.
The combined use of Si and Mn to produce a sintered steel is mentioned in the literature, but no description is provided concerning the procedures to be followed in producing such a sintered steel. Sintered steels produced by the combined use of Si and Mn, moreover, are stated "not to bring any surprising results." See, Findeisen, G., Hewing, J., "Copper and Nickel Containing Sintered Steels Including Further Alloying Additives", Industrie-Anzeiger, Volume 92 (1970), pages 241-244 and 431-434, especially page 434.